This invention relates to a laser light generating apparatus for radiating a continuous light of a short wavelength based upon the outgoing light of a laser light source.
An optical recording medium is employed for recording/reproduction of data such as video or audio signals. Although a large quantity of data may be recorded on an optical recording medium, it is desirable for an optical recording medium to have a recording capacity much larger than the current recording capacity.
For realizing an optical recording medium of a larger recording capacity, it is required to raise the information recording density of the information recording medium. One of the techniques of increasing the recording density is to shorten the wavelength of the laser light employed for high density recording.
Recently, a demand has been raised for a laser light of the ultraviolet wavelength as a short wavelength laser light. In particular, the laser light of the ultraviolet wavelength in the vicinity of the wavelength of 355 nm is desired most strongly. For generating the laser light of this wavelength range, there is known a method of generating third harmonics of the infrared wavelength used as the fundamental wavelength.
If, for generating the third harmonics (THG) by a laser light radiating device, Nd:YAG or Nd:YVO.sub.4, a solid laser, is employed as a laser medium, the laser light of the fundamental wave generated by exciting the laser medium by a light pumping method and the second harmonics produced on wavelength conversion of the fundamental laser light are frequency-mixed for generating the third harmonics of the fundamental laser light, that is the laser light in the vicinity of the wavelength of 355 nm within the UV range.
The laser light in the vicinity of the wavelength of 355 nm is a laser light generated with the pulse mode. The reason the THG incurs the mode of pulse oscillation is that, when the outgoing laser light of the fundamental wavelength is frequency-mixed with the second harmonics, the laser light of the third harmonics is generated by a single pass in which the laser lights are passed only once through a non-linear crystal. The result is that the non-linear conversion of third harmonics is limited to only one pass resulting in pulse oscillation of the laser light of third harmonics.
Since the laser light radiated in the vicinity of the wavelength of 355 nm is a pulsed light, such laser light is difficult to apply to optical recording or optical display where continuous light is necessitated.
As a method for raising the non-linear conversion efficiency by the continuous light by the laser light generating apparatus, there has been proposed a method of arranging a non-linear crystal within the light path of the light resonator.
This method achieves short harmonics generation (SHG) by using a resonator having high finesse and high transmittance for the light of the wavelength equal to one-half the fundamental wavelength and having suppressed losses in level at the fundamental wavelength.
However, for generating the third harmonics as the continuous light, the fundamental laser light wave with a frequency of 1 .omega. and the second harmonics laser light by SHG with a frequency of 2 .omega. need to be resonated at the time of additive frequency mixing of the two laser light beams. In addition, a mirror constituting the resonator needs to be of low reflection with respect to the third harmonics. Furthermore, wavelength dispersion of the resonator needs to be synchronized for resonating the fundamental wavelength light generated by the laser light source with the second harmonics within the same resonator.